1. Field of the Invention:
The present invention relates to an apparatus for casting silicon by an electromagnetic induction.
2. Background of the Related Art:
A method is known in which an electrically conductive bottomless crucible 6 (FIGS. 7a and 7b) divided into a plurality of parts in a circumferential direction thereof is placed in an induction coil 7 and a material 20 is gradually drawn downwards from said bottomless crucible 6 with melting within the bottomless crucible 6 and subsequent solidification. This method is hereinafter refereed to as continuous casting by electromagnetic induction.
In the continuous casting method by electromagnetic induction, since the bottomless crucible 6 is divided into a plurality of parts in the circumferential direction thereof, respective divided parts 16 are electrified by an electric current flowing through an induction coil 7 so that an electric current is generated in a material 20 within the crucible 6 to heat and melt the material 20. A repulsion is simultaneously generated between the electric current flowing through the divided parts 16 of the crucible 6 and the electric current flowing through the material 20 to maintain the material 20 in a condition that it is not brought into contact with the crucible 6.
However, in the continuous casting method by electromagnetic induction, the molten material 20 within the crucible 6 unavoidably enters gap 17 (FIGS. 8a and 8b) between the divided parts 16 of the crucible 6 and solidifies. This phenomenon is called the insertion phenomenon, which leads to difficulty in pulling down and discharge of the solidified material 20 from the inside of the crucible 6.
Accordingly, in practice, the continuous casting method by electromagnetic induction has been in the form where a slag is placed between the crucible 6 and the material 20 within the crucible 6. The continuous casting method by electromagnetic induction in which slag is put between the crucible 6 and the material 20 within the crucible 6 is called the inducto-slag melting method and has been mainly used for the melting and casting active metals such as titanium. In this method, the slag serves as a cushion material and an insulating material between the crucible 6 and the material 20 within the crucible 6. See (Note) P.G. CLITES and R.A.BEALL: Proc. the Fifth International Conf. on Electroslag and Special Melting Technology, (1975); P477.
On the other hand, an anisotropic solidified silicon lump used as a material for a solar cell and the like has been industrially produced by a method in which silicon is melted in a bottomed crucible in an atmosphere inert to molten silicon and then poured into a mold having a predetermined temperature gradient in the vertical direction to solidify.
However, in such a method of casting silicon, the contamination of the silicon with impurities from the crucible and mold can not be avoided. In addition, in order to suppress such contaminations, it is required that the crucible and mold are formed of special highly pure materials. This leads to a remarkable increase of casting cost and complexity of heating facilities for the mold. Furthermore, the crystallization of the silicon progresses simultaneously from both a bottom surface of the mold and a side surface of the mold, so that such the method is not preferable in view of crystallography.
In view of the above described problems, it was considered that the continuous casting method by electromagnetic induction capable of maintaining the crucible under the condition that it is not brought into contact with the material within the crucible is suitable for the casting of silicon.
The continuous casting method by electromagnetic induction has been considered for a long time but not practically used in an industrial scale until the inducto-slag melting method was developed, in view of the balance against the power source facilities and electric-power cost. However, a small-sized but large-capacity power source facility has been provided as the technology has remarkably progressed recently, to reduce the electric-power cost. The continuous casting method by electromagnetic induction is again being considered as a method of casting silicon.
In the continuous casting method by electromagnetic induction, as above described, the material is not brought into contact with the bottomless crucible and in the case where it is used for the casting of silicon, the silicon can be completely prevented from being contaminated with impurities. If silicon is not contaminated with the impurities from the crucible, the grade of the material for the crucible can be lowered and the cost of facilities can be remarkably reduced in cooperation with the elimination of the mold. Thus, the continuous casting method by electromagnetic induction can continuously produce a large-sized and high-quality silicon cast lump inexpensively by combining it with the large-capacity power source apparatus. In addition, the crystallization from the side wall of the crucible can be suppressed, so that the continuous casting method by electromagnetic induction is preferable also in view of the crystallography.
The continuous casting method by electromagnetic induction is shown in Japanese Patent Laid-Open No. Sho 61-52962. In addition, in this method the temperature gradient during the cooling after the solidification of silicon is large, which generates a great thermal strain in the solidified silicon cast lump and the thermal strain leads to the generation of many cracks and crystalline defects in the silicon cast lump. As a result, the quality of the silicon cast lump is not satisfactory.
The insertion phenomenon can be solved by the supply of great electric power, but this cannot prevent the deterioration due to the thermal strain, which is a fatal disadvantage for a semi-conductor material. In addition, if the insertion phenomenon is overcome by the great electric power, the power cost is increased, so that it is desired to suppress the insertion phenomenon without using great electric power, if possible.